Demand for small high-capacitance capacitors with low impedance at high frequencies is growing in response to the increasing digitization of electric appliances. Conventional capacitors used at high frequencies include mica capacitors and multi-layered ceramic capacitors. Downsizing of these capacitors, however, is difficult to reconcile with maintenance of large capacity.
Typical large-capacitance capacitors are electrolytic capacitors such as aluminum electrolytic capacitors and aluminum or tantalum solid electrolytic capacitors. The extremely thin anodized layer which becomes the dielectric of these capacitors allows them to achieve large capacitance. On the other hand, thin anodized layers are fragile, and thus these capacitors require electrolytic substances to allow them to repair themselves.
For example, in aluminum electrolytic capacitors, an etched positive electrode and aluminum foil forming the negative electrode are wound together, providing a separator in between, and the separator is impregnated with liquid electrolyte. This liquid electrolyte has ion conductivity and large specific resistance, resulting in large losses which significantly degrade the frequency characteristics as well as the impedance and temperature characteristics. Moreover, leakage of liquid and evaporation of solvent are inevitable, resulting in reduced capacitance and increased loss as time passes. Solid tantalum electrolytic capacitors use manganese oxides as their electrolyte for improved temperature characteristics and reduced secular change of capacity and loss. However, the relatively high specific resistance of manganese oxide results in poor impedance frequency characteristics compared to multi-layered ceramic capacitors and film capacitors. To counter this disadvantage, solid aluminum and tantalum electrolytic capacitors with high electric conductivity employing conjugated double bond conductive polymer are proposed.
In general, conjugated double bond conductive polymers, typically poly-aniline and poly-thiophene, are prepared by chemical oxidative polymerization and electro-polymerization.
Application of electro-polymerization puts a limit on mass production because conductive polymer are formed on an electrode in a film. In contrast, application of chemical oxidative polymerization is free from such restrictions, and allows a large volume of conductive polymer to be relatively easily produced by reacting a monomer using an oxidizing agent.
In order to use such conductive polymer as the negative electrode conductive layer of a solid electrolytic capacitor, it is important to provide the conductive polymer with high environmental stability and high dielectric layer repairing ability.
The conductive polymer consists of polymer as a main component and dopant. Selection of dopant anionic material or introduction of an appropriate substitute group for monomer has conventionally been applied to improve environmental stability of the conductive polymer.
In particular, a number of studies have been carried out on the use of thiophene as the monomer by introducing the ethylene dioxy group to its β, β′ positions (3, 4 positions) for creating a conductive composition with high environmental stability.
It is generally thought that the dielectric layer of the conductive polymer used in the solid electrolytic capacitor is self-repaired by the change of the conductive polymer to insulating polymer as a result of the joule heat generated by the current flowing to the dielectric layer defect portion. Conventionally, the dielectric layer formed by anodization is thickened to achieve a solid electrolytic capacitor with high withstand voltage, using conductive polymer for the negative electrode conductive layer. In other words, the anodization voltage is often increased to thicken the dielectric layer. Another approach is the use of anionic compounds which have high dielectric layer repairing ability as dopants for in-situ formation of the negative electrode conductive layer consisting of conductive polymer.
Still another approach is to carry out in-situ polymerization in the presence of phenol derivatives to form a negative electrode conductive layer consisting of conductive polymer to provide high dielectric layer repairing ability and thus to achieve a capacitor with high withstand voltage. However, the strategy of increasing the anodization voltage during the formation of the dielectric layer to allow a sufficient margin against the applied voltage with the aim of securing a high withstand voltage thickens the dielectric layer in proportion to the rated voltage, causing the undesirable result of reducing the capacitance of the capacitor.
In addition, it is difficult to achieve both sufficiently high environmental stability, particularly high heat resistance in the air, and high dielectric layer repairing ability at the same time using a single dopant, although the properties of the polypyrrole are changeable by types of dopant used.
Accordingly, several dopants are often used in some approaches, but it is still difficult to achieve a conductive polymer with high heat resistance and high layer repairing ability because doping ability varies among dopants and the control of individual doping ratio is difficult.
The use of additives for improving the layer repairing ability has also been studied, but most of the additives that are added to the polymerizing solution for in-situ polymerization are removed during the rinsing for removal of polymerization residue, and thus sufficiently high layer repairing effects are not achieved.
The repairing ability of conductive polymer using anionic materials, which have a high anodization layer repairing ability, as dopants, still confers a low level of repairing ability combined with low withstand voltage.
However, there is strong market demand for solid electrolytic capacitors using conductive polymer with withstand voltages the same with those of general electrolytic capacitors.